Fusion cast refractories are refractories which are manufactured by melting the raw materials in an electric arc furnace and then pouring the batch into a mold and allowing it to solidify to form the desired finished shape. Fusion cast refractories are normally formed from alumina usually along with zirconia and silica. Proportions of other minerals such as chromite may be added to control the crystalline structure and physical properties of the end products. The main use for fusion cast refractories is in the glass industry as a lining for glass melting furnaces, due to the chemical stability, impermeability, and resistance to corrosion and erosion at the working temperatures in the glass melting furnaces. Fusion cast refractories are also used in various other industrial applications such as in certain types of chemical reactors.
One type of mold which is used for the casting of fusion cast refractories is formed from white silica sand with an organic-based binder system. Other particulate materials that are used are olivine sand, zircon sand, carbon sand, alumina grain and fused grain such as fused silicon oxide. A variety of organic materials may be used for such binders including oil binders, furan binders and phenolic-urethane binders. When the molten refractory material is poured into a mold with an organic binder, there may be a reaction between the molten refractory material and the organic binder which will evolve gases such as hydrogen, oxygen and nitrogen and also form other gases such as carbon monoxide and water. A result of the formation of these gases is that porosity defects occur in the solidified refractory material. These defects may vary from only minor subsurface pores, which still have acceptable density, to extremely unacceptable pore patterns which occur through the entire casting. Factors which have an effect on the extent of the pores are the pouring time and temperature and the resulting exposure of the mold cavity to thermal degradation. Also, the chemical composition of the organic binder has an effect. For example, with a polyisocyanate-polyol binder, gas evolution and defect formation are enhanced when the ratio of polyisocyanate to polyol is greater than one, which is usually the case.